It is known that plants can be grown in mineral wool growth substrates. Such growth substrates are typically provided as a coherent plug, block, slab or mat/blanket and generally include a binder, usually an organic binder, in order to provide structural integrity to the product.
Typically, the growth process of the plant is managed in two stages: a first stage managed by a “propagator” in which the plant is grown from seed; and a second stage managed by a “grower” during which the plant is sustained and any harvest taken. For example, in the case of the tomato plant, the propagator may plant individual tomato seeds in cylindrical plugs having a thickness in the order of 25-30 mm and a radius of around 20-30 mm. After germination of the seed, the propagator places the plug within a cuboid block to allow further growth of the root system and the plant. The individual plant within the block is then nursed until a stage when it can be transferred from the propagator to the grower.
Although typically only a single plant is provided in each block, it is possible for multiple plants to be provided in a single block. In some examples, a single plant in a block is split into two by splitting a stem during an early phase of growth, resulting in two plants sharing a single root system. In another alternative, multiple plants may be grafted together and grown within a single block.
The use of a separate plug and block by the propagator is not essential for all plants, but has been described, for example, in European patent application EP2111746, as providing a number of advantages. In particular, the small size of the plug allows more regular watering of the plant in the initial stage without saturating its substrate.
After they are received from the propagator, the grower places a number of blocks on a single slab of mineral wool to form a plant growth system. The slab of mineral wool is typically encased in a foil or other liquid impermeable layer except for openings on an upper surface for receiving the blocks with the plants and a drain hole provided on the bottom surface.
During subsequent growth of the plant, water and nutrients are provided using drippers which deliver a liquid containing water and nutrients to the system either directly to the blocks or to the slabs. The water and nutrients in the blocks and slabs is taken up by the roots of the plants and the plants grow accordingly. Water and nutrients which are not taken up by the plant either remain in the substrate system or are drained through the drain hole.
There is a desire to use water and nutrients as efficiently as possible during the growing process. This is both for cost and environmental reasons. In particular, the nutrients are expensive to obtain, while waste water containing such nutrients is difficult to dispose of due to environmental legislation. The desire to avoid such waste is matched by a desire to improve plant growth conditions, and thereby to increase the yield and quality of fruit obtained from plants in this manner.
The use of mineral wool itself provides significant benefits in this regard as compared to traditional soil-based growing methods, but there is an ongoing requirement to further improve these characteristics. In particular, there is a conflicting desire to both produce more and consume less in plant growth processes. That is, a greater yield from the plants is desired while at the same time reducing the amount of water and/or nutrients that are used. In practice, existing growing methods and/or substrates provide limitations on both these aspects.
Important qualities of plant growth systems in this context include their water retention, re-saturation and water/nutrient distribution. The water retention reflects the quantity of water that can be retained by the system while the water distribution reflects the location within the slab of the water and nutrients that are present. The re-saturation refers to the tendency of newly added liquid solution to add to the water and nutrient levels of the substrate rather than replace existing solution or be spilled.
Particular considerations which affect water retention, water distribution and re-saturation include the effect of gravity, which tends to force water downwards and thus towards the drain hole, and capillary effects which can cause water to be drawn upwards. In practice, the slabs are typically provided on a slight slope, with the drain hole located at the lowest end of the bottom surface, helping to ensure that gravity forces the water towards the drain hole. In addition to gravity and capillary effects, the flow resistance of the medium should be considered, which has the effect of preventing water passing through the slab from the drippers to the drain hole. Overall, if root and plant development is to optimised, then it is necessary to ensure that optimal conditions are found in the region of the substrate in which the roots are growing.
As would be expected, poor water retention leads to water being lost, and thus wasted, through the drain hole. The water distribution is also important since it is necessary for the water within the slab to reach the plant roots. For example, when a plant has recently been placed on the slab, the roots will extend only into the upper regions of the slab. Thus if the majority of water sinks to the bottom of the slab due to the effects of gravity, then the plant may not receive sufficient water and/or nutrients. In particular, in order to ensure that the plant roots in the top region of the slab are sufficiently watered, it may be necessary for the grower to provide excessive water to the slab so that the lower regions contain more water than is required, leading to greater wastage through the drain hole and extra costs. Excessive water levels can also increase the risk of fungal growth which may damage the plant.
An example of the difficulties that arise due to the imbalance in water concentration occurs as a result of seasonal variations. As summer turns into autumn, the days grow shorter and the amount of sunlight provided to the plant growth systems reduce. As a result, the level of evaporation of water from the system also reduces. It is desirable therefore to provide less water to the plant growth system as the requirement to replace evaporated water is reduced. However, since the water is tends to flow to the bottom of the slab a reduction in the water provided to the system risks drying out the top of the slab. To avoid this risk, unnecessary water is often provided, leading to waste. These conditions occur particularly in winter or early spring, and are often particularly acute when the plant/blocks are initially placed on the slabs. At this stage it is essential that the top of the slab is wet enough for the plant roots to begin growth within the slab but this can often lead to significant wastage of unused water and/or nutrients in the lower part of the slab, which can, for example, be lost through a drain hole.
Another factor in the plant growth is the retention and distribution of nutrients. Although the nutrients are typically introduced with the water, they will not necessarily be distributed and retained by the slab in the same way. The nutrients typically comprise dissolved salts comprising nitrogen, phosphorus, potassium, calcium, magnesium and similar elements. The nutrients are dissolved in the water and their movement through the slab is affected by processes such as advection, dispersion and diffusion. Advection is the movement of nutrients with the water flow through the slab, dispersion is the mixing of nutrients that occurs as they travel through complex pore structures in the slab, and diffusion relates to random movement of particles within the slab and the statistical tendency this has to reduce concentration gradients.
As with the water itself, it is important that the nutrients reach the plant roots. If nutrients are poorly distributed, or are lost from the slab, then excess nutrients may be required in the slab as a whole for the plant to receive the nutrients it requires. This is of course a waste of nutrients.
Another consideration that plays a role in plant growth on man made substrates is the nutrient refreshment efficiency. This relates to whether the introduction of new nutrient solution will flush out existing nutrients in the slab. In some circumstances, it may be desirable to change the nutrient concentration within the slab during the growth process. The ability to do this will depend on whether existing nutrients can effectively be replaced through the whole slab or at least the region of the slab in which root growth takes place. Moreover, in some examples a build up of nutrients if they are not replaced can reach levels which can cause dehydration or at least non-ideal for plant growth.
The problems identified above relate at least partly to the inherent properties of the slab. However, further difficulties and inconsistencies arise because of the action of the plants themselves. In particular, the plant root systems do not take water or nutrients from the slabs uniformly. This difficulty is particularly pronounced when multiple blocks are provided in each plant growth system. For instance, the different plants in the system are likely to develop differently and have different requirements. This increases the complexity in providing the correct water and nutrient content to each plant.
One attempt to improve the distribution of water and nutrients in a substrate is described in European patent application EP0300536. In this document, a system is described in which multiple blocks are provided on a single slab and a capillary system is used to apply a constant suction pressure across the slab. The aim of this suction pressure is to draw more water in to areas that become relatively dry, thereby increasing the uniformity of the water distribution across the slab.
In practice, there are a number of drawbacks with the system described in EP0300536. In particular, while there may be some benefit in terms of the uniformity of the water distribution across the slab in the horizontal plane, this does not provide significant benefits in terms of the vertical variation in water content that has been found. Moreover, the system of EP0300536 is relatively expensive to implement.
Notwithstanding these efforts, there is an ongoing requirement to improve the distribution and retention of water and nutrients in the slab in order to allow greater performance efficiency and cost-effectiveness in plant growth methods and thereby to secure a more sustainable way of growing. Existing techniques often result in the loss and/or overfeeding of water and/or nutrients due to the difficulties in controlling the distribution and retention of a feed solution in the slab in such a way to satisfy the requirements of plant growth.